With self-propelling work machines, such as bulldozers, tracked vehicles, or other self-propelling off-road vehicles for construction sites, mines and the like, electric drives having at least one electric motor have been used in recent times to leverage the typical advantages of such electric drives with respect to hydrostatic drives, such as their higher efficiency and easier maintenance. Considerably lower operating costs can also be achieved by operating at substantially lower powers due to the increased efficiency of such drives. The electric drive can be utilized both as a traction drive for driving at least one chain drive of an undercarriage as well as for driving a main work unit such as the milling drum of a surface miner.
A generator can be provided to supply energy for operating the electric drive. In one example, said generator may be drivable by an internal combustion engine, for example in the form of a diesel engine, a gasoline engine, or a gas engine. The engine may drive one or more auxiliary devices via the generator. In other words, the generator is a load on the engine. Also, when operating, the one or more auxiliary devices are also a load on the engine. For example, not only the power generator, but also a hydraulic unit, such as a pump or fan of the hydraulic unit, can be driven by the internal combustion engine so as to hydraulically drive other adjustment actuators of hydraulic components. With a bulldozer, for example, the adjustment and/or lifting device for the trenching shovel can be driven by means of such hydraulic actuators via the engine (and the generator). With dump trucks, as another example, the dump body can be rocked up and down by means of a hydraulic actuator via the engine (and the generator).
One example of a bulldozer having such a drive concept comprising an electric drive is shown in U.S. Pat. No. 7,950,481. Therein an electric motor is centrally arranged so that its drive power can be used to drive different elements via a differential. Excess electrical energy which is generated by the generator when an associated internal combustion engine is not utilized to capacity is stored in a battery in order to be able to transfer additional electrical energy in the sense of a boost function to the electric motor when the latter requires a particularly high power, such as on the starting up of the machine. If, conversely, the work machine is to be braked, mechanical brakes in the form of spring pre-loaded disk brakes which can be hydraulically ventilated are actuated. However, the inventors have recognized an issue with such systems. Depending on the size of the work machine and on its function/purpose, such brakes may have to be dimensioned large in order not to overload or overheat on intensive braking procedures over an extended duration, such as can be the case with bulldozers constantly moving backward and forward or with fully loaded dump trucks traveling downhill. In still other examples, the amount of electrical energy that can be received at the battery may be limited due to constraints such as battery temperature, battery state of charge, battery load, etc. In such a scenario, the excess electrical energy is dumped and cannot be productively used.
In another example, U.S. Pat. No. 8,395,335 B2 describes an electric drive system for off-road trucks in which the electrical drive energy is provided by an internal combustion engine which drives a generator. During braking operations, electrical motor braking power provided by the electric motors is transferred to the generator to reduce the fuel consumption of the internal combustion engine. Excess electrical motor braking power is furthermore transferred past the internal combustion engine to electrical auxiliary units to drive these electrical units electrically and is finally dissipatively reduced or “burnt”, e.g., converted into heat, via electrical braking resistors in the form of a so-called grid box. The distribution of the electrical motor braking power, however, requires a relatively complicated control system while taking account of the electrical energy usable at the auxiliary units. In addition, the thermal load arising at the grid box has to be taken into account.
It is the underlying object of the present disclosure to provide an improved work machine of the initially named kind as well as an improved method for braking such a work machine which avoid disadvantages of the prior art and further develops the latter in an advantageous manner. An energy-efficient braking with sufficient decelerations may be made possible using a braking apparatus which is of a simple design and is easy to control.
In one example, the above mentioned issues may be overcome by a work machine comprising a self-propelling work machine, the machine comprising an internal combustion engine; an electric drive having at least one electric motor, a generator drivable by the internal combustion engine and supplying the electric drive with electrical energy, and at least one auxiliary unit coupled to the internal combustion engine; a braking apparatus configured to provide a regenerative braking by the electric drive; a feedback apparatus for feeding back electrical motor braking power from the electric motor to engine via the generator, and a controller with computer readable instructions stored on non-transitory memory for adjusting the electrical load of the at least one auxiliary unit of the engine based on the electrical motor braking power fed back to the internal combustion engine via the generator. The above mentioned issues may also be addressed by a method for braking a work machine comprising: responsive to a braking demand, generating electrical power from regenerative braking at an electric motor of an electric drive of the hybrid vehicle; transferring the generated electrical power from the electric motor to a generator coupled to an engine; and adjusting one or more of an engine fueling, an engine speed, and an electrical load applied by an auxiliary device on the engine via the generator based on the generated electrical power.
As an example, a braking power applied on an internal combustion engine may be controlled by varying the load applied on the engine by at least one auxiliary unit (or device) which is connected to the internal combustion engine (herein also referred to as power pick-up of the auxiliary unit). The auxiliary unit applying load on the engine may be, for example, in the form of a fan, a cooling apparatus, or a pump. The auxiliary unit applying load on the engine may include the unit applying a load on the generator wherein the generator is driven by the engine. In particular, the auxiliary unit may draw a current from the generator, wherein the electrical current or load at the generator is generated via the spinning engine. A control apparatus may automatically increase or decrease the load applied by the at least one auxiliary unit on the generator, and thereby on the engine, based on the electrical motor braking power fed back to the internal combustion engine (that is, the regenerative braking power generated via the electric motor which is then fed back to the generator). Additionally or optionally, the power consumption of or load applied by the at least one auxiliary unit may be varied by the control apparatus based on the operating state (e.g., the engine speed) of the internal combustion engine acted on by fed back motor braking power. The electrical motor braking power that is being generated and fed back can be measured or determined directly by determining an electrical characteristic such as the voltage or current of a feedback apparatus, for example via an inverter. Alternatively, the electrical motor braking power can be inferred or indirectly determined based on a characteristic accompanying the motor braking power such as a torque which the generator generates while being acted on by the fed back motor braking power. The power pick-up or load applied by the auxiliary unit can, however, not only be controlled in dependence on the motor braking power itself, but also in dependence on the operating state of the internal combustion engine acted on by the fed back motor braking power and/or on the auxiliary unit connected thereto, for example based on a speed of the internal combustion engine.
In this way, the regenerative motor braking power can be varied, thereby enabling the total braking power to also be varied (e.g., increased) in a controlled manner. The technical effect of such a control of the regenerative braking power is that the application, and therefore the wear and tear, of any mechanical brakes included in the drive system can be delayed. In addition, an even more efficient operation of the work machine can be achieved. For example, a power output of a fan or a cooling apparatus of the vehicle can be ramped up to beyond a degree required to cool corresponding units (that is, to provide more than the requested cooling) so that during vehicle operation on a subsequent ascent or on a level path, the auxiliary unit can be switched off for longer or can be operated at a lower power than required.
As another example, an auxiliary unit such as a pump can be operated in a dissipative manner with a higher output, and thereby a higher load applied on the generator and the engine (herein also referred to as a higher power pick-up) in order to increase the regenerative motor braking power, for example by increasing the flow rate of the pump, such as by connecting a flow resistance.
A hydraulic pump which is not required for vehicle travel operation (that is, for vehicle propulsion) and which either conveys in idle circulation during driving or is swiveled to a conveying quantity of zero can in one example be used as the auxiliary unit with whose assistance the retard capacity (that is, the ability of the generator to receive the motor braking power generated at the motor during regenerative braking) of the retard system comprising the internal combustion engine and the auxiliary unit can be variably controlled. To increase the retard or braking capacity of the retard system during a braking operation, the power pick-up of the hydraulic pump conveying in idle circulation during driving can advantageously be increased in that a flow resistance is successively increased in circulation. For this purpose, for example, a pre-controllable pressure-relief valve can be used which is correspondingly controlled by the control apparatus when a higher retard/braking performance and thus a higher flow resistance is required.
If the hydraulic pump used as the auxiliary unit is swiveled to conveying a quantity of zero during driving, the power pick-up of the hydraulic pump can advantageously be increased in that the hydraulic pump is successively swiveled out in circulation against a constant flow resistance, and against a fixedly set pressure relief valve.
One example of such a hydraulic pump not required in driving operation includes the hydraulic pump for the pressure circuit of a bulldozer, the pressure circuit used to adjust the dozer blade.
Alternatively or additionally to such a hydraulic pump, a cooling apparatus, for example a cooling fan, can be used as the auxiliary unit whose power pick-up (or load applied by the cooling fan on the generator coupled to the engine) can be ramped up when the required braking power increases, and which can be ramped down when the required braking power decreases. It will be appreciated that the load applied by the auxiliary unit on the generator may be varied while the engine is operated in a speed control mode so that the engine speed can be maintained while reducing the fueling of the engine, providing fuel economy and engine performance benefits.
The control apparatus can in particular increase the power consumption of at least one such auxiliary unit before a mechanical brake is used and/or further motor braking power of the at least one electric motor is dissipatively reduced, e.g. burnt, for example via a braking resistor.
The control apparatus in this respect advantageously provides that the desired braking effect is primarily achieved by regenerative braking via the electric motor(s) and the electrical motor braking power generated in thus process is applied on the internal combustion engine and on the auxiliary units connected thereto (via the generator) until the retard capacity of the internal combustion engine and of the auxiliary unit(s) is essentially completely exhausted. The retard capacity being exhausted includes the generator not being able to accept any more braking power (such as in the form of an electric current or voltage) from the electric motor(s) and maintaining engine speed control. In one example, the retard capacity may be limited due to a battery coupled to the engine system being too hot or due to the state of charge of a battery coupled to the generator being too low.
The braking energy or the electrical motor braking power provided by the at least one electric motor is advantageously primarily fed back (in the form of electric current or voltage) to the internal combustion engine via the generator when the vehicle is coasting. The generator then converts the electrical motor braking power into mechanical drive power for the internal combustion engine which is used for driving one or more secondary power consumers (or auxiliary units) such as fans, coolers or pumps connected to the internal combustion engine, and for overcoming the drag resistances of the internal combustion engine. In this way, the regenerative braking power can be used to supply the generator with current to support the electrical load of the auxiliary units, while reliance on the engine to supply the generator with current to support the electrical load of the auxiliary units is reduced. As a result, while the engine is operated in a speed-control mode, the engine speed can be maintained with engine fueling gradually reduced and while the auxiliary loads continue to be supported via the generator.
If one or more of the motor braking power applied on the internal combustion engine and on the auxiliary units connected thereto also exceeds a degree compatible with the internal combustion engine, or if the auxiliary power requests on the engine after ramping up the power pick-up of the auxiliary units reaches a predefined operating state, or if the internal combustion engine or the at least one auxiliary unit reaches a predefined operating state under the effect of the fed back motor braking power, a mechanical brake can automatically be connected or electrical energy can be transferred to the braking resistor to reduce a further increase of the electrical motor braking power applied on the internal combustion engine. In one example, due to the engine operating with speed control, the amount of braking power that can be fed back to the engine may be limited. In addition, the amount of braking power that can be fed back to the generator and the system battery may be limited due to conditions such as elevated battery temperature, elevated battery state of charge, etc. During such conditions, by actuating the mechanical brake, the desired braking effort can be provided with reduced wear and tear of the mechanical brakes. For example, the connection or coupling of the braking resistor and/or of the mechanical brake can advantageously be ramped in gently as required. In one example, the braking force applied from the braking resistor and/or the mechanical brake can be successively increased so that a smoother transition from a braking without a mechanical brake to a braking with a mechanical brake, and vice versa, can take place. In one example, the transition can take place in the manner of a blending procedure in a gently transiting manner without a retard burst (that is, without a sudden spike or drop in net braking force, such as while maintaining a target steady rate of braking force application). The braking force of the mechanical brake, but also the braking power which is applied on the at least one auxiliary unit can be gently varied and controlled, in particular regulated, while taking account of the braking power already applied on the internal combustion engine, in order to more accurately provide a desired braking force predefinable by the driver. In one example, the controller may determine a deficit between the net braking power requested and the braking power available via the electric motor braking. Further, the controller may determine a target rate of braking force application. Based on the deficit and the target rate, the amount and rate of braking force applied via the mechanical brake and/or the braking resistor can be adjusted so that the net braking force is provided. As also elaborated herein, the load of the auxiliary units can also be increased (such as by increasing the output of an auxiliary fan or pump) when the retard capacity of the engine is reached.
The electrical braking resistor can in this respect advantageously be used for brief durations for the reduction of braking power peaks, for example only for some seconds, to reduce voltage peaks occurring in the voltage circuit. In permanent operation, the system can advantageously work without the braking resistor.
Braking advantageously only takes place using the mechanical brake when the motor braking power fed back to the internal combustion engine reaches the retard capacity of the internal combustion engine and of auxiliary units which may be connected thereto. In this way, mechanical brake and braking resistor usage is reduced, improving component life.
An increasing electrical motor braking power which is generated by the at least one electric motor can in particular first be applied on the internal combustion engine with an increasing braking power, for example by increasing actuation of a brake generator (e.g., increasing magnitude of actuation of the brake generator, increasing the electrical current directed to the brake generator, etc.) and/or by an increasing slope (that is, increasing rate of actuation or current application at the brake generator), with the fuel supply to the internal combustion engine being successively reduced until the internal combustion engine no longer consumes any fuel at a constant speed. In this way, engine speed feedback control is used for adjusting engine fueling (e.g., a fuel-based engine speed control method is applied) so as to automatically regulate the fuel while the engine concurrently accepts however much current the braking motors are able to provide back to the generator (up to the maximum speed limit of the engine) via the regenerative braking effort. Once this limit is reached, the controller may then further adjusts the output of the auxiliary units (e.g., fans). As the fed back electrical motor braking power increases further, the internal combustion engine can advantageously be revved up beyond a constant engine speed desired (for the current operating conditions) until a maximum permitted or maximum desired engine speed of the internal combustion engine is reached, with the named revving up of the internal combustion engine advantageously taking place with a blocked fuel supply. The maximum permitted or maximum desired engine speed of the internal combustion engine may have a predefined value and may be dependent on vehicle operating conditions such as a battery state of charge, ambient temperature, engine temperature, etc. In one example, the braking power can be received at the engine via the generator and used to increase the engine speed to an upper limit while maintaining engine fueling. Then, once the upper limit of engine speed is reached, further braking power can be received at the engine via the generator and used to maintain the engine speed at or below the upper limit while reducing engine fueling (and/or while increasing the output of the auxiliary units). In this way, the regenerative braking effort can be maximized while performing engine speed control.
If the predefined maximum speed of the internal combustion engine is reached, the control apparatus starts to ramp up the load applied on the engine (or power pick-up) by the at least one auxiliary unit to be able to place further motor braking power on the internal combustion engine via the generator and on the auxiliary unit connected thereto. The ramping up of the electrical load applied by the auxiliary unit in this respect advantageously takes place gently in the sense of a blending procedure to ensure a gentle increase in the braking power. For example, the rate of ramping up of the electrical load applied on the engine via the auxiliary unit may be determined based on regenerating braking effort available, the net braking effort requested, speed limits and constraints of the auxiliary unit (e.g., a speed limit of an auxiliary fan, an output limit of an auxiliary hydraulic pump) as well as battery conditions such as battery temperature and battery state of charge. In this respect, the magnitude of a desired braking force is taken into account, e.g. the increased load applied by the auxiliary unit is only ramped up so much that the provided retard power (provided by the regenerative motor braking) is not larger than the desired retard power.
The control apparatus therefore advantageously provides a plurality of braking stages which can be connected one after one another (e.g., sequentially) to provide the desired or required braking power. Initially or primarily, electric motor braking power is applied on the internal combustion engine without ramping up the power pick-up of the auxiliary unit and without connecting mechanical brakes so as to operate the internal combustion engine energy efficiently with a fuel supply which is reduced as much as possible. Then, upon reaching a retard capacity of the internal combustion engine (beyond which the engine cannot absorb excess energy from regenerative braking) or on reaching the compatibility limit of the application of the electrical motor braking power, the power pick-up of the auxiliary unit is ramped up in a further stage. In this respect, within the aforesaid first braking stage in which the electrical motor braking power is only or at least primarily applied on the internal combustion engine, the fuel supply is in this respect initially reduced in a first sub-stage with a substantially constant internal combustion engine speed for so long until the fuel supply is completely cut off. Once the fuel supply is cut off, a revving of the internal combustion engine is permitted in a second sub-stage. In one example, regenerative motor braking power may be received at the engine via the generator and used to maintain the engine speed while reducing engine fueling. Then, once the engine fueling is below a threshold amount of fuel (e.g., all fueling to the engine is stopped), further braking power can be received at the engine via the generator and used to raise the engine speed to or below an upper limit. In addition, further braking power can be received at the engine via the generator and used to raise the output of one or more auxiliary units, such as to increase a fan speed or a pump output.
The named control apparatus is configured in a further development of the present disclosure such that the optionally present mechanical brakes remain unactuated or released for so long until the desired or required braking power can be applied on the internal combustion engine and on the auxiliary units connected thereto via electrical motor braking and feeding back of the motor braking power, in particular for so long until the fed back motor braking power does not exceed a predefined limit value and/or the internal combustion engine acted on by the fed back motor braking power and/or the auxiliary unit connected thereto does/do not leave a predefined operating state or operating state range, in particular does/do not exceed a predefined engine speed.
The control apparatus can in particular connect the mechanical brake in dependence on the engine speed of the internal combustion engine, specifically, only when the speed of the internal combustion engine reaches a predefined maximum speed and the at least one auxiliary unit is operated to a pre-defined maximum power pick-up. The control apparatus can for this purpose be connected to a speed detection device (e.g., a speed sensor) which provides the speed of the internal combustion engine and to a determination device for determining the operating state and/or the power pick-up of the at least one auxiliary unit. For example, a current, voltage sensor, or pressure sensor may be coupled to the auxiliary unit and/or a torque sensor may be coupled to a shaft of the auxiliary unit for determining an electrical load of the auxiliary unit on the engine.
The control apparatus can furthermore comprise an engine controller (such as a controller with computer-readable instructions stored on non-transitory memory which when executed can perform the following steps or functions) for reducing the fuel supply to the internal combustion engine in a manner which initially reduces the fuel supply while maintaining a constant speed of the internal combustion engine. That is, fueling is adjusted for engine speed control. In particular, the engine controller may send a signal to engine cylinder fuel injectors to reduce fueling to an increasing degree such that with an increasing application of electrical motor braking power on the internal combustion engine the fuel supply is progressively driven to zero and in so doing the engine speed is kept constant and/or at least at a predefined minimal speed, for example an engine idling speed.
The control apparatus can generally be realized in different manners, for example in the form of software which is executed by a central control computer or in the form of a plurality of software modules which are executed in separate calculation modules or in the form of one or more separate or interlinked control modules.
The present disclosure will be explained in more detail in the following with reference to associated drawings.